In broad area lasers, the lateral divergence of the emitted radiation increases with increasing output power, as a result of the increase in thermal energy within the structure at higher powers. The laser is heated partly by the current flow into the laser, and partly by the re-absorption of the emitted light. Since the semiconductor materials utilized in the laser structure exhibit temperature-dependent refractive indices, a thermally-induced waveguide is formed as the device's operating temperature increases (a significant aspect of high power operation). As a result, lasing of (unwanted) higher-order lateral modes with increasing power is created, causing ever-increasing lateral divergence.
The scale of the refractive index contrast formed by such a thermally-induced waveguide is in the range of about 10−4 to 10−3. The increasing lateral divergence associated with this change in refractive index reduces the ability of the output beam to focus within the desired dimensions, limiting its use in applications such as materials processing, for example, that require a high beam quality.
One prior art approach to minimizing problems associated with the thermally-induced waveguide is described in U.S. Pat. No. 8,233,513 entitled “Broad-Area Edge-Emitting Semiconductor Laser with Limited Thermal Contact” and issued to D. Schleuning et al. on Jul. 31, 2012. Here, the broad area laser diode is mounted on a large heat-sink, where the heat-sink is formed to include a pair of parallel grooves, creating a ridge that has a width about equal to the width of the emitter region. The laser diode is mounted so as to contact the ridge portion of the heat-sink, providing a path for the transfer of thermal energy away from the emitter region of the device. The grooves function to confine heat flow to the ridge, thus minimizing thermally-induced refractive index contrast.
Another attempt at overcoming the problems associated with thermally-induced waveguides is based on the incorporation of high refractive index anti-guiding regions within the laser diode structure itself, as described in detail is U.S. Pat. No. 8,537,869 entitled “Broad Area Diode Laser with High Efficiency and Small Far-Field Divergence”, issued to P. Crump et al. on Sep. 17, 2013. While able to reduce the unwanted higher-order lateral modes, the inclusion of these anti-guiding regions within the active area of the device requires the modification of conventional process steps associated with the fabrication of laser diodes, adding to the cost and complexity of the final structure.